Recent advances in AM broadcast transmission and reception, such as AM stereo, have spurred the development of wider bandwidth, "higher fidelity" AM receivers. These receivers represent a significant improvement in terms of audio frequency response over their narrow-bandwidth counterparts. Under very good signal conditions (typically within 50 miles of the transmitter), these receivers can reproduce transmitted frequency components in the 6 to 12 kHz range without adverse effects from adjacent channel transmitter occupancy at +-10 kHz. However, under reception conditions where adjacent channel signal strengths are comparable to the desired signal, the wider bandwidth receivers pass objectionable 10 kHz "whistle" and "monkey chatter". In addition, random noise which becomes noticeable under very weak signal reception (independent of any adjacent channel interference) makes a wide receiver bandwidth unacceptable under "deep fringe" conditions. This has led receiver manufacturers to include fixed 10 kHz notch filters and selectable wide/narrow IF bandwidth in typical AM stereo receivers.
Typical implementations of 10 kHz notch filters and selectable IF bandwidths in current AM receivers have significant disadvantages and performance deficiencies. For example, notch filters typically constructed with passive inductive, capacitive, and resistive components, have Q value limitations of 7 to 8 because of component tolerances and allowable temperature drift. This attenuated frequency spectra includes much more than 10 kHz, and can significantly limit reproduction of transmitted audio components in the 8-12 kHz range. Also, the maximum IF bandwidth of the receiver is dictated not by audio fidelity, but by maintenance of a reasonable RF protection ratio under low to moderate adjacency conditions. It is therefore a listenability compromise in terms of both ultimate "fidelity" and adjacent channel interference rejection. Listener selectable IF bandwidths provide protection from adjacent channel interference, but are poor in terms of user interface because they offer only two manually selectable bandwidth alternatives. As signal strength decreases, background noise tends to limit the signal to noise ratio (S/N) of high frequency modulation components in a given listening situation. The listener is often forced to switch to the narrower IF bandwidth even though no adjacencies may be present.
The main disadvantage of 10 kHz notch filters and IF bandwidths in current AM receivers is, then, their fixed or manual nature, constrained by adequate RF protection ratios. The listener is not afforded totally "automatic" adjustment of audio response to which he is accustomed in typical automotive FM receivers. Also, the fidelity limitations and the lack of flexibility previously mentioned do not allow a receiver's audio response to be continuously optimized.